This invention relates generally to methods for agglomerating or pelletizing mineral ore concentrate. More specifically, this invention relates to methods for agglomerating or pelletizing mineral ore concentrate using water soluble, high molecular weight polymer binder systems in water in-oil emulsions, in oil dispersions or as a dry powder. 2. Description of the Prior Art
It is customary in the mining industry to agglomerate or pelletize finely ground mineral ore concentrate so as to further facilitate the handling and shipping of the ore.
Mineral ore concentrates can include iron oxides, copper oxides, barytes, lead and zinc sulfides, and nickel sulfides. Agglomerates of coal dust and nonmetalic minerals used to make bricks or ceramics are also formed. Agglomerate forms can include pellets, briquettes, and sinters.
Methods of pelletizing mineral ore concentrate are frequently used in mining operations where the ore is a low grade iron ore. Examples of low grade iron ores are taconite, hematite, and magnetite. Numerous other low grade ores exist wherein pelletizing of the ground particles is beneficial to the handling and shipment of the mineral ore. After the mineral ore has been mined, it is frequently ground and screened to remove large particles which are recycled for further grinding. Typically, an ore is passed through a 100 mesh (0.149 mm) screen. The screened mineral ore is known as a "concentrate".
For example, taconite mineral ore concentrate after grinding and screening has an average moisture content of between about 6 to about 10 percent. The moisture content of the mineral ore concentrate can be selectively altered. The moisture content affects the strength of the pellets that are formed later in the process.
After screening, the mineral ore concentrate is transported on a first conveyor means to a balling drum, balling disc, or another means for pelletizing mineral ore concentrate. Prior to entering the balling apparatus, a binding agent is applied or mixed into the mineral ore concentrate. Commingling the binding agent with the mineral ore concentrate occurs both on the conveyor means and in the means for pelletizing. The binding agents hold the mineral ore concentrate together as pellets until after firing.
Balling drums are apparatus comprising long cylindrical drums which are inclined and rotated. The mineral ore concentrate is simultaneously rotated about the balling drum's circumference and rolled in a downward direction through the drum. In this manner the mineral ore concentrate is rolled and tumbled together to form roughly spherical shaped pellets. As the pellets grow in size and weight they travel down the incline of the drum and pass through the exit of the drum at which point they are dropped onto a second conveyor means which transports them to a kiln for firing. Inside the balling drum, different factors influence the mechanisms of union of the mineral ore concentrate. These factors include the moisture content of the ore, the shape and size of the mineral ore particles, and the distribution of concentrate particles by size. Other properties of the mineral ore concentrate that influence the pelletizing operation include the mineral ore's wettability and chemical characteristics. The characteristics of the equipment used, such as its size and speed of rotation, can affect the efficiency of the pelletizing operation. The nature and quantity of the agglomerating or binding agent used in the concentrate is also a factor that determines part of the efficiency of the pelletizing operation.
The formation of agglomerates begins with the interfacial forces which have a cohesive effect between particles of mineral ore concentrate. These include capillary forces developed in liquid ridges between the particle surfaces. Numerous particles adhere to one another and form small pellets. The continued rolling of the small pellets within the balling apparatus causes more particles to come into contact with one another and adhere to each other by the capillary tension and compressive stress. These forces cause the union of particles in small pellets to grow in much the same manner as a snowball grows as it is rolled.
After the balling operation, the pellets are formed, but they are still wet. These pellets are commonly known as "green pellets", though taconite pellets, for example, are usually black in color. Green pellets usually have a density of about 130 lb/ft.sup.3 in sizes between about 1/2 inch and about 3/8 of an inch in diameter. The green pellets are transported to a kiln and heated in stages to an end temperature of approximately 2800.degree. F. After heating, fired pellets are extremely hard and resist cracking upon being dropped and resist crushing when compressed.
Two standard tests are used to measure the strength of pellets whether the pellets are green pellets or fired pellets. These tests are the "drop" test and the "compression" test. The drop test requires dropping a random sampling of pellets a distance, usually about 18 inches or less onto a steel plate. Said pellets are dropped a number of times until the pellets crack. The number of drops to crack each pellet is recorded and averaged. Compression strength is measured by compressing or applying pressure to a random sampling of pellets until the pellets crumble. The pounds of force required to crush the pellets is recorded and averaged. These two tests are used to measure the strength of both wet and fired pellets. The drop and compressive test measurements are important because pellets, proceeding through the balling drum and subsequent conveyor belts, experience frequent drops as well as compressive forces from the weight of other pellets travelling on top of them.
Thermal shock resistance is a factor which must be taken into consideration in any process for agglomerating mineral ore concentrate. Increases in a pellet's thermal shock resistance improve that pellet's ability to resist internal pressures created by the sudden evaporation of water when the pellet is heated in a kiln. If the pellet has numerous pores through which the water vapor can escape thermal shock resistance is improved. If the surface of the pellet is smooth and continuous without pores the pellet has an increased tendency to shatter upon rapid heating. This causes a concurrent increase in the amount of "fines" or coarse particles in the pelletized mineral ore. A binder which increases the pores formed in a pellet improves that pellet's ability to resist thermal shock.
Bentonite is used as a binding agent in the pelletizing operations for taconite ore concentrate. Bentonite produces a high strength pellet having an acceptable drop strength, compressive strength, and thermal shock resistance. Bentonite has the disadvantage of increasing the silica content of the pellets that are formed. Silica decreases the efficiency of blast furnace operations used in smelting of the ore. For this reason bentonite requires a higher energy expenditure than do organic binders.
Binding agents other than bentonite have proven to be useful as binders. These agents include organic binders such as poly(acrylamide), polymethacrylamide, carboxymethylcellulose, hydroxyethylcellulose, carboxyhydroxyethylcellulose, poly(ethylene oxide), guar gum, and others. The use of organic binders in mineral ore pelletizing operations is desirable over the use of bentonite because organic binders do not increase the silica content of pellets and they improve the thermal shock resistance of the pellets. Organic binders burn out during pellet firing operations and cause an increase in the porosity of the pellets. Firing conditions can be modified to improve fired pellets' mechanical properties for organic binder systems.
Some organic binders used in mineral ore pelletizing operations are dissolved in an aqueous solution which is sprayed onto the mineral ore concentrate prior to entering the balling. This application of an aqueous solution increases the moisture content above the natural or inherent moisture content of the mineral ore concentrate, which requires a greater energy expenditure during the firing operation of the pellets. This increased moisture content also causes an increased likelihood of shattering due to inadequate thermal shock resistance during firing. Pellet formation is improved with the use of organic binders, but the drop strength and compression strength of the pellet are frequently below that desired or achieved with bentonite.
Other binders commonly used for agglomerating mineral ore concentrate include a mixture of bentonite, clay and a soap, Portland cement, sodium silicate, and a mixture of an alkali salt of carboxymethylcellulose and an alkali metal salt. The agglomerates made from these binding agents frequently encounter the problems described above of insufficient pellet strength or insufficient porosity for the rapid release of steam during induration with heat. Additionally, these binding agents are usually applied to a mineral ore concentrate in aqueous carrier solutions or as dry powders. Aqueous carrier solutions increase the amount of energy required to fire the pellets and increase the incidence of pellet shattering due to inadequate thermal shock resistance.
U.S. Pat. No. 3,893,847 to Derrick discloses a binder and method for agglomerating mineral ore concentrate. The binder used is a high molecular weight, substantially straight chain water soluble polymer. This polymer is used in an aqueous solution. The polymers disclosed as useful with the Derrick invention include copolymers of acrylamide as well as other polymers. The Derrick invention claims the use of polymers in an "aqueous" solution. The use of water as a carrier solution for the binding agents increases the moisture of the agglomerates or pellets that are formed. The higher moisture content increases the energy required to fire the pellets and can increase the rate of destruction of the pellets during induration due to the rapid release of steam through the agglomerate.
The industry is lacking a method for agglomerating mineral ore concentrate utilizing low water content non-bentonite binder systems, such as water soluble, high molecular weight polymer binder systems in water-in-oil emulsions, dispersions in oil, or dry powders. This invention provides pellets formed from the mineral ore concentrate of high mechanical strength properties.